In a large mixing bowl with a beater or stand mixer, beat the softened butter on high until completely smooth and creamy, about 2-3 minutes.Add powdered sugar 1 cup at a time, beating on low to incorporate then increasing to high speed. After each cup is well incorporated, repeat the process.
½ cup unsalted butter, 4 cups powdered sugar
Next, add champagne, vanilla, and optional food coloring. Mix well to combine, scraping down the sides with a rubber spatula as needed.
¼ cup champagne, 1 teaspoon vanilla extract, 2 drops red food coloring
Notes
For a more intense flavor, heat the champagne in a small sauce pan over medium-high heat. Once boiling turn it down to medium and cook down the champagne until it is reduced to a syrup. Set aside until it cools before adding to the buttercream.